TGR5 receptor is a G-protein-coupled receptor that has been identified as a cell-surface receptor that is responsive to bile acids (BAs). The primary structure of TGR5 and its responsiveness to bile acids has been found to be highly conserved in TGR5 among human, bovine, rabbit, rat, and mouse, and thus suggests that TGR5 has important physiological functions. TGR5 has been found to be widely distributed in not only lymphoid tissues but also in other tissues. High levels of TGR5 mRNA have been detected in placenta, spleen, and monocytes/macrophages. Bile acids have been shown to induce internalization of the TGR5 fusion protein from the cell membrane to the cytoplasm. Kawamata et al. 2003, J. Bio. Chem., 278, 9435. TGR5 has been found to be identical to hGPCR19 reported by Takeda et al. 2002, FEBS Lett. 520, 97-101.
TGR5 is associated with the intracellular accumulation of cAMP, that is widely expressed in diverse cell types. While the activation of this membrane receptor in macrophages decreases pro-inflammatory cytokine production, (Kawamata, Y., et al. J. Biol. Chem. 2003, 278, 9435-9440) the stimulation of TGR5 by BAs in adipocytes and myocytes enhances energy expenditure (Watanabe, M. et al. Nature. 2006, 439, 484-489). This latter effect involves the cAMP-dependent induction of type 2 iodothyronine deiodinase (D2), which by, locally converting T4 into T3, gives rise to increased thyroid hormone activity. Consistent with the role of TGR5 in the control of energy metabolism, female TGR5 knock-out mice show a significant fat accumulation with body weight gain when challenged with a high fat diet, indicating that the lack of TGR5 decreases energy expenditure and elicits obesity (Maruyama, T., et al. J. Endocrinol. 2006, 191, 197-205). In addition and in line with the involvement of TGR5 in energy homeostasis, bile acid activation of the membrane receptor has also been reported to promote the production of glucagon-like peptide 1 (GLP-1) in murine enteroendocrine cell lines (Katsuma, S., Biochem. Biophys. Res. Commun. 2005, 329, 386-390). On the basis of all the above observations, TGR5 is an attractive target for the treatment of disease e.g., obesity, diabetes and metabolic syndrome.
In addition to the use of TGR5 agonists for the treatment and prevention of metabolic diseases, compounds that modulate TGR5 modulators are also useful for the treatment of other diseases e.g., central nervous diseases as well as inflammatory diseases (WO 01/77325 and WO 02/84286). Modulators of TGR5 also provide methods of regulating bile acid and cholesterol homeostasis, fatty acid absorption, and protein and carbohydrate digestion.
Relatively few examples of TGR5 agonists have been described in literature. Recently, 23-alkyl-substituted and 6,23-alkyl-disubstituted derivatives of chenodeoxycholic acid (CDCA), such as the compound 6α-ethyl-23(S)-methyl-chenodeoxycholic acid shown below, have been reported as potent and selective agonists of TGR5 (Pellicciari, R.; et al. J. Med. Chem. 2007, 50, 4265-4268).

In particular, the methylation (S-configuration) at the C23-position of natural bile acids (BAs) confers a marked selectivity to TGR5 over FXR (farnesoid X receptor) activation, whereas the 6α-alkyl substitution increases the potency at both receptors. Some examples of other TGR5 agonists include 6-Methy1-2-oxo-4-thiophen-2-yl-1,2,3,4-tetrahydro-pyrimidine-5-carboxylic acid benzyl ester (WO004067008, Takeda Chemical Industries LTD, Japan, 2004) and oleanoic acid (Sato, H. et al. Biochem. and Biophys. Res. Commun. 2007, 362, 793-798; Ito, F. et al. WO2004067008, 2004). More recently, the first synthesis of enantiomeric chenodeoxycholic acid (CDCA) and lithocholic acid (LCA) has allowed access to studying the specificity of the interaction of natural BAs with TGR5 (Katona, B. W. et al. J. Med. Chem. 2007, 50, 6048-6058).
Recently developed TGR5 agonists have also provided for the first time a pharmacological differentiation of genomic versus nongenomic effects of BAs and have also allowed for informative structure-activity relationship studies, for example, the presence of an accessory binding pocket in TGR5 has been found to play a pivotal role in determining ligand selectivity (See, Pellicciari, et al. J. Med. Chem. 2007, 50, 4265-4268). In this context, the availability of more potent and selective TGR5 modulators is necessary to further identify additional features affecting receptor activation and characterize the physiological and pharmacological actions of this receptor in order to better understand its relationship to the prevention and treatment of disease.
To this end, of particular interest were the biological and physicochemical properties of the compound cholic acid (CA), which has the structure shown below:
Cholic acid is a primary bile acid in human and many animal species, also reported as one of the main components together with bilirubin of Calculus Bovis, a highly valued traditional Chinese medicine (Chen, X., Biochem. Pharmacol. 2002, 63, 533-541). Cholic acid (CA) differs from chenodeoxycholic acid (CDCA) and its derivatives described above by the presence at C-12 of an additional alpha-hydroxyl group oriented on the polar side of the molecule. This “minor” structural difference accounts for the remarkably different physicochemical and biological features of these two bile acids. With respect to CDCA, protonated CA is about 4-fold more soluble and relatively less detergent as a result of its hydrophobic/hydrophilic balance and polarity. Moreover, CA is devoid of activity toward FXR receptor (EC50>100 μM) while showing moderate agonistic activity on TGR5 (EC50=13.6 μM). As an even more important consideration, it was previously reported that the pharmacological administration of CA at 0.5% w/w in diet-induced obese mice efficiently prevents and treats metabolic syndrome (Katsuma, S., Biochem. Biophys. Res. Commun. 2005, 329, 386). While this study provided interesting results related to the endocrine functions of bile acids, the high dosage required (0.5% w/w) still limited the proof of concept concerning the therapeutic relevance of TGR5 in the context of metabolic diseases, since the modulation of other and unknown targets could not be ruled out at that dose. An additional issue was also the risk associated with testing a high dose of CA in clinical trials due to the production of the toxic secondary metabolite BA DCA via extensive and efficient intestinal bacteria 7α-dehydroxylation (Nagengast, F. M., Eur. J. Cancer, 1995, 31A, 1067).
Thus, there is a need for the development of TGR5 modulators for the treatment and/or prevention of various diseases. The present invention has identified compounds that modulate TGR5 as well as methods of using these compounds to treat or prevent disease.